With the rapid development of global industrialization, pollutants such as harmful heavy metals and organic substances go into the soil through various channels, causing soil pollution. Soil pollution not only affects agriculture, but also leads to groundwater pollution and also impacts human health negatively. Thus remediation for contaminated soil is a technique that highly needs development at present. Chlorobenzenes are physicochemically stable, uneasy to be hydrolyzed chemically and degraded biologically in soil, and are a kind of hydrophobic chlorinated organic pollutants widely existing in the environment. They can be irritating to human skin and mucosa, and can also be suppressive and narcotic to the central nerve system, causing human poisoning and endangering human health. In addition, chlorobenzenes often settle in soil for a long time, which causes serious pollution to soil and water. Therefore, research on degradation of chlorobenzenes and their residues in soil is of great importance. Although there are many kinds of remediation techniques, few of them are of universal adaptability.
A permeable reactive barrier (PRB) remediation technology is a kind of in-situ passive remediation technology. Its basic principle is to establish a permeable reaction zone along the flow direction or a direction perpendicular to the water flow direction, and the reaction zone is filled with active materials (such as zero-valent iron, activated carbon, and so on). When the water flow goes through the PRB under its own hydraulic gradient, there will be chemical reactions between the pollutants and the wall materials during which the pollutants are removed, so as to achieve the purpose of environmental remediation. Once installed, except in some cases where there is a need to replace the wall reaction materials, PRB will incur almost no other operating and maintenance expenses. Thus it is a technology that costs little and can be used for a long time. The main principle of an electrokinetic remediation technology is to apply a direct current (DC) voltage at the two ends of the contaminated soil to form an electric field gradient, then the pollutants in the soil will be migrated to the electrodes under the effects of electromigration, electroosmosis and electrophoresis in the electric field, so as to clean the soil and remove the pollution. The main targets of this technology include inorganic pollutants (such as lead, cadmium, chromium, zinc and other heavy metals), polar organic pollutants (such as phenol, chlorophenols, 4-chlorophenols, etc.) and non-polar organic pollutants (such as benzene, toluene, and xylene, etc.) and radioactive substances. The technology features the characteristics of high efficiency and cleanness. It can deal with pollutants at depth of up to 10 to 20 meters and has wide applicability.
Because of differences in the composition of soils, and in the types and properties of pollutants, especially in the case of multiple pollutions, it's difficult to reach the ultimate target with only a single remediation technology. In the electrokinetic remediation technology, due to a large amount of H+ and OH− produced in cathode and anode in the electrochemical reaction process, in cases where there are no additional restrictions and measures, these ions will move to the cathode and anode by electromigration, electroosmosis and diffusion electrophoresis under the action of the electric field, until the two ions meet and neutralize, where they cause a change of the pH value, and where the entire operation zone can be divided into an acidic area and an alkaline area. Therefore, how to control the pH value has become the focus of the electrokinetic remediation technology. The PRB technology also has certain limitations. First, along with the continuous deposition of matters, such as organic substances, at the permeable reactive wall, the passive treatment will lose activity; therefore the equipment must be replaced regularly. Moreover, the present study on PRB is still insufficient, and there has been no best way to deal with multi components of multiple organic matters. The combined EK-PRB remediation technology is a new and environment-friendly remediation technology. This technology mainly combines the electrokinetic remediation technology and the PRB technology together in the remediation of contaminated soils, by which the organic substances with high toxicity are moved to the electrodes, where they react with the filling materials in the permeable reactive wall so that the pollutants can be degraded into low-valence metal ions, and organic matters with low toxicity. Thus this technology not only can remove pollutants but also can reduce their toxicity.
As an in-situ remediation technology that combines the advantages of the electrokinetic and PRB technologies, the combined EK-PRB technology can handle not only heavy metal pollutants, but also organic pollutants, such as chlorobenzenes. Additionally, the technology is also resistant to external interfering factors, capable of avoiding the addition of environment-unfriendly substances into the soil, and incurs much less remediation cost than other methods. Thus, an effective EK-PRB remediation technology is highly demanded to treat the groundwater and soils contaminated by organic pollutants such as chlorobenzenes.